1. Field of the Invention
The present invention relates to polarization selection or switching and more particularly to a high speed polarization switch which selects between predetermined polarization modes for electromagnetic radiation transferred through a waveguide. The invention further relates to a polarization switch using diode arrays for selectively shorting predetermined polarization modes.
2. Related Technical Art
It is often desirable to select between or preferentially monitor different polarization modes in received electromagnetic radiation signals. This is often done as a means for extracting or inferring information from received signals through isolation of specified polarization modes in which the information is embedded. Also, inter-signal interference may be decreased by attenuation of some polarization modes.
In the area of high frequency radar, radiation reflected by most targets is generally found to have consistent polarization characteristics dependent upon the reflection medium. That is, when reflecting a radar signal comprising multiple polarization modes, metallic targets with extensive planar surfaces typically return substantially mono-polar radiation, or radiation having a single polarization mode. Planar metallic surfaces provide isolation into singular polarization modes due to the nature of radiation interactions with the surface. On the other hand, general background or "clutter" type reflection sources tend to return radiation that is more randomly distributed among two or more polarization modes. Therefore, polarization mode detection can be applied in the analysis of radar signals to help differentiate between "true" targets and "false" targets, especially in the presence of undesirable clutter.
Polarization mode detection is a useful feature to incorporate in advanced target acquisition systems to allow monitoring of polarization modes of reflected or received radiation to determine changes as a function of seeker or detector direction. However, detecting polarization modes in received radiation requires filtering or removal of some modes while the relative strength of remaining modes is detected.
In the case of circularly polarized radiation, polarization mode removal is generally accomplished by transferring received radiation through a polarization selective switch that shorts out or shunts undesired modes. Such switches typically comprise a series of gratings which are aligned with one mode and act to shunt or short out that mode. The gratings can be implemented using a series of fine wires, wire mesh, metal grids, or slotted plates. However the grids must be mechanically oriented and redirected or isolated, to dynamically select between different polarization modes. Mechanical operation imposes a significant penalty in terms of response time. It can take on the order of one millisecond or more to mechanically change grid orientation. At the same time, a mechanical approach also requires a substantial amount of working volume and complexity to implement. This may impact reliability and reproducibility.
For commercial and military applications, volume, speed and reliability are key factors in the design of advanced radar detection and analysis equipment. Advanced avionics designs demand minimum volume and weight in radar equipment to meet changing airframe aerodynamics and weight limitations. Other types of radar systems including space borne and naval applications also require ever smaller volumes. At the same time, there is a desire for faster response in mode selection and analysis. Current polarization mode selection techniques or approaches are not satisfactory for many of these applications and provide limited flexibility in equipment design.
What is needed is a method or apparatus for polarization mode selection which operates at high speeds, in minimum volumes, and with a minimum of complexity.